In a transmit/receive unit of such a telecommunication system it is often necessary to provide a duplexing filter blocking a first signal frequency F.sub.a, e.g. that of an outgoing carrier, and to pass without significant attenuation a second, closely adjoining signal frequency F.sub.b, such as that of an incoming carrier. Filters of this type must be highly selective to eliminate possible interferences between the two signal channels; they should also introduce as small an insertion loss as possible.
Filters conventionally used for this purpose, especially in the microwave range, include coaxial series-resonant and parallel-resonant cavities. Such structures have good electrical characteristics but are relatively bulky and require a high degree of precision in their manufacture, which makes them expensive to produce.
Less costly combinations of lumped-constant band-pass and band-stop filters are satisfactory only for relatively large frequency differences F.sub.a -F.sub.b.